1. Field of the Invention
The present invention relates to a process for producing 2,6-dialkylnaphthalene (DAN) and, in particular, 2,6-dimethylnaphthylene (2,6-DMN) from a mixture which contains alkylnaphthalene or naphthalene.
2. Discussion of the Background
In the manufacture of high performance polyester resins such as polyethylene naphthalate polymer (PEN) or polybutyrene naphthalate polymer (PBN), 2,6-DMN is used as a precursor of 2,6-naphthalene dicarboxylic acid. This is because 2,6-DMN is easily oxidized to 2,6-naphthalene dicarboxylic acid, when compared to other precursors such as 2,6-diisopropylnaphthalene or 2-methyl-6-isobutyrylnaphthalenes. There are many applications for PEN, e.g., films and bottles, such as long time recording type video film, Advanced Photo System, hot fill containers, refillable bottles and tire codes. PEN has good physical properties in strength, thermal resistance and gas barrier properties. Typical PBN applications include electronics, insulators and car parts, PEN and PBN have heretofore been too expensive, however, to effectively expand their markets due to the limited commercially viable processes for producing 2,6-DMN.
There have been many proposals for preparing 2,6-DMN. U.S. Pat. No. 4,795,847 (Weitkamp et al.) describes a process for the preparation of 2,6-dialkylnaphthalene by alkylating naphthalene or 2-alkyl-naphthalene with an alkylating agent in the presence of a zeolite (specially ZSM-5) as a catalyst.
U.S. Pat. No. 5,001,295 (Angevine et al) describes a process for preparing DMN by using 2-monomethylnaphthalene (MMN) and naphthalene as a feedstock and a synthetic zeolite (MCM-22) as a catalyst, and it shows MCM-22 catalyst is more effective than ZSM-5 in alkylation of 2-MMN and naphthalene.
However, the above methods provide only unit operation (i.e. batch) for alkylation of 2-MMN, which is an expensive feedstock and is not commercially available in a large amounts.
U.S. Pat. No. 4,990,717 (Sikkenga) and 5,073,670 (Sikkenga et al.) describe a multi-step process to produce 2,6-DMN from o-xylene and butadiene, which consists of:                1) preparation of 5-(o-tolyl)-pentene-2(OTP) by alkenylation of o-xylene with butadiene in the presence of a catalyst such as an alkali metal catalyst,        2) preparation of 1,5-dimethyltetralin (1,5-DMT) by cyclization of OTP in the presence of a catalyst such as platinum and copper on an ultra stable zeolite catalyst;        3) preparation of 1,5-dimethylnaphthalene (1,5-DMN) by dehydrogenation of 1,5-DMT in the presence of a catalyst such as platinum and rhenium and gamma alumina; and        4) preparation of DMN mixture which is rich in the desirable 2,6-DMN, 1,6-DMN and 1,5-DMN by isomerization of 1,5-DMN in the presence of a catalyst such as a beta-zeolite catalyst.        
If a process for separating 2,6-DMN from a DMN mixture were combined with the above steps, a complete process to produce purified 2,6-DMN could be provided.
As multiple steps complicate a process plant and increase the cost, it is not clear that the conventional processes could provide a process suitable for an economical preparation of purified 2,6-DMN.
In addition, it is very difficult to separate 2,6-DMN from other isomers by conventional separation methods such as distillation and cooling crystallization because;                1) There are very small differences in the boiling points of DMN isomers, and, in particular, between 2,6-DMN and 2,7-DMN wherein the difference in boiling points is only 0.3 C, and it is nearly impossible to separate 2,6-DMN by distillation.        2) The cooling of DMN isomer mixture solution of 2,6-DMN purification forms a precipitate of very fine 2,6-DMN crystals in suspension, and thus separation of the 2,6-DMN is extremely difficult.        
Koide et al U.S. Pat. No. 4,992,619 reports a method for separating a methyl derivative of naphthalene from a mixture of materials in high purity by crystallization under pressure.
Moritoki et al U.S. Pat. No. 4,784,766 reports a pressure crystallization apparatus.
Accordingly, new and more efficient methods for commercially preparing dialkylnaphthalenes are sought.